Advanced Diagnostics
Mission Statement
Inertial confinement fusion (ICF) and high-energy-density (HED) physics experiments involve phenomena that occur on timescales measured in picoseconds and spatial scales measured in micrometers (or microns). Observable information is conveyed in photons with energies ranging from a few electron volts (eV) from visible light to gamma rays with energies of million eV (MeV), and in charged and neutral particles from fusion reactions. The development and innovation of new instruments and techniques ensured a steady progress in ICF. HED has been dependent for decades on the development and innovation of new instruments and techniques to measure the observables with increasing temporal, spatial, and energy resolution, and with the ability to gather more data in a single experiment.
Neutron Time-of-Flight (nTOF) Detectors
The basic ingredients of a neutron time-of-flight (nTOF) detector are a scintillator material block coupled to a fast photomultiplier tube (PMT) and digital recording system such as an oscilloscope, see figure.1 More information on the OMEGA nTOF detector can be found in the short video below.
The neutrons interact with the scintillator material and produce a visible photon signal that is detected as a function of time with the PMT and the digitizer recording the current created in the PMT. The spectrum of the neutron energies is deduced from the time of flight between source and detector; hence the name of the detector. The spectrum of the primary and the scattered fusion neutrons is one of the key measurements used to infer the implosion performance. The detector is located a known distance from the point source (the hot spot formed in the implosion) and since the neutron pulse from the implosion is short (~100 ps in duration), the energy of the neutrons can be determined by the time-of-flight technique. As the neutrons traverse the flight path distance, the neutron pulse becomes broader with the highest-energy neutrons arriving before the lower-energy neutrons. A measurement of the arrival time relative to the implosion time gives the velocity or the energy of the neutrons. A sufficient fast instrument response function (IRF) is required, which is ensured by a fast decay time of the scintillation process, a fast PMT response, and a high-bandwidth transient digitizer. A fast instrument response minimizes the distortion of the incoming primary DT neutron signal by the detector IRF and therefore enables accurate measurements of the primary DT neutron energy spectrum. Several nTOF detectors are placed around the OMEGA target chamber to provide multiple lines of sight that are required to study the 3-D effects of cryogenic DT implosions on OMEGA.2
Nuclear Activation Diagnostics
Neutron measurements are essential for studying hot, dense, inertial-confinement fusion (ICF) plasmas based on hydrogenic fuels (e.g., deuterium, deuterium–tritium) that reach conditions sufficient for producing copious fusion neutrons. These neutrons, as penetrating reaction products, carry information about the conditions under which they were created. Neutron-activation measurements, although time integrated, play a crucial role, specifically, in determining the neutron yield from the ICF plasma and in determining whether the neutron emission is isotropic (and thus consistent with thermonuclear equilibrium conditions). Several types of activation materials have been found useful (e.g., In-115, Cu-63, Zr-90), depending on the plasma reactions involved and the anticipated neutron yield. On OMEGA, coincidence counting using twin NaI detectors and gamma spectroscopy from high-purity germanium detectors (HPGe) are the two approaches used to measure the nuclear yields.
Coincidence Counting Detector
A DT neutron yield diagnostic based on the reactions, 63Cu(n,2n)62Cu(β+) and 65Cu(n,2n) 64 Cu(β+), is used to infer the DT yield on the Omega Laser Facility. The induced copper activity is measured using a NaI γ –γ coincidence system.
High-Purity Germanium Detector (HPGe)
The method chosen for measuring the time integrated DD yield is with neutron-induced activation of indium isotopes. In this method, the gamma-ray activity produced by the 115In(n, n’)115mIn is measured to infer a neutron yield. The gamma-ray line activity of the produced isomers 115mIn (336.23 keV) is measured with a high-resolution gamma-ray detector.
Spherical Crystal Imager3
A shaped Bragg crystal-imaging system is used to obtain radiographs in direct-drive implosion experiments. A quartz crystal, cut along the 1011 planes for a 2-D spacing of 0.6687 nm, is set up for the Si Heα line at ~1.865 keV (0.664 nm) with a Bragg angle of 83.9° or 6.1° from normal incidence. The crystal was mounted by direct-optical contact on an aspheric glass substrate to correct for the optical distortions caused by the oblique incidence. The crystal has a major radius of curvature of 500 mm and is placed 267 mm from the implosion target. The image is recorded on a detector located ~3.65 m from the target for a magnification of ~15×. The available solid angle to place the backlighter foil is quite limited since the backlighter target must not intercept any of the 60 beams pointed at the implosion target. Because the backlighter laser intensity must be kept as high as possible, the 500-µm square backlighter was placed at a distance of 5 mm from the implosion target. A fast target insertion system (FASTPOS) inserts the backlighter target 100 ms after the shroud that protects the layered cryogenic target from ambient thermal radiation has been removed. FASTPOS also acts as the direct line-of-sight (LOS) and has additional collimators to suppress background. The backlighter is driven by the OMEGA EP laser with up to 1.5 kJ of energy at a pulse duration of 20 ps. An x-ray framing camera (XRFC) head with an exposure time of ~40 ps is used to record the image. The XRFC is triggered by an ultrastable electro-optical trigger system with a jitter of ~1.5-ps rms specifically developed for this application.
Fresnel Zone Plates4
Experiments carried out with a continuous-wave (cw) x-ray source on the OMEGA and OMEGA EP Laser Systems have used a Fresnel Zone Plate (FZP) to obtain x-ray images with static spatial resolutions of around 1.5 mm. the FZP focus condition, where p and q satisfy the focus equation and form a first-order (1st) image of an object O. The x-ray backlighting source is located behind the object at B. A zeroth-order (0th) contribution surrounds the first-order image. The FZP may or may not include a central block (CB) to reduce the I0 contribution. The implementation of the FZP diagnostic on OMEGA and OMEGA EP uses x-ray film, an image plate, a framing camera, or a charge-coupled device to detect the images of objects backlit at energies in the multi-keV range using x-ray line emission from laser-illuminated metal foils. The available spatial resolution is determined by a combination of the spectral content of the x-ray source and the detector resolution limited by the image magnification. Typical magnifications for current FZP setups on OMEGA and OMEGA EP are in the range of 14 ft to 42 ft. High-speed framing cameras have been used to obtain frame times with the FZP imager as short as 30 ps.
The figure on the right shows the FZP focus condition from a first-order (I1st) image of an object O. The x-ray backlighting source is located behind the object at B. A zeroth-order (I0th) contribution surrounds the first-order image. The FZP may or may not include a central block (CB) to reduce the I0th contribution. Typical magnifications for current FZP setups on OMEGA and OMEGA EP are in the range of 14× to 42×.
Spatially Resolved Electron Temperature (SR-TE)5
The spatially resolved electron temperature (SR-TE) diagnostic has been developed to provide ~20 time-integrated hot-spot images of direct-drive cryogenic implosions, with 18.5× magnification at x-ray mean energies of ~10 to 20 keV. These differentially filtered images will be used to assess spatially integrated hot-spot electron temperature, hot-spot mix, and hot-spot temperature profile. The images are recorded using image plates and differential filtering (nominally in four sections) at the image plane with Al, Ti, or other user specified foils. The apertures for SR-TE are positioned outside the instrument’s vacuum system and remain at the target chamber conditions. They are located 109.5 mm from target chamber center and shielded from debris by a 20-mil Be filter and a single 1-mm Al collimator. The initial design of the imaging apertures is a honeycomb array of precision 125-µm diam apertures with <3-µm variation in circularity and spacing (at aperture plane) of 820 µm (General Atomics). From the images, the central, brighter hot-spot emission is isolated from the background coronal emission. The hot-spot emission is then deconvolved based on the precision circular apertures. With this coded aperture, or penumbral imaging technique, the impact of background noise is reduced and thus previously inaccessible hard x-ray energies become accessible. Resolution is expected to be detector limited to ~7 µm, however, implications of the numerical inversion will be assessed in the context of use.
The Single Line-of-Sight Time-Resolved X-ray Imager (SLOS-TRXI)6
The SLOS-TRXI provides time-resolved images of the hot spot of direct-drive ICF implosions with a 40-ps temporal resolution and a 10-µm spatial resolution. This is achieved by combining an electron pulse-dilation imager with a nanosecond-gated sensor. X-rays emitted from the hot spot in the 4- to 9-keV energy range around the time of peak compression are imaged through a pinhole array onto a photocathode. The photons produce electrons via the photoelectric effect. A time-varying voltage is applied at the photocathode, causing electrons born earlier in time to have higher kinetic energies than electrons born later in time. The electron pulse enters a 75-cm drift tube, which is held at a constant potential. The differing velocities of the electrons cause the electron pulse to elongate and be temporally magnified. The electrons are then detected on multiple frames of a hybrid complementary metal-oxide semiconductor (hCMOS) sensor, producing four time-resolved images of the hot spot over a total recording time of about 160 ps. More information on SLOS-TRXI can be found in the short video below.
DAD Detector7
The diagnostic for areal density (DAD) uses Cherenkov radiation in quartz to detect gamma rays. It is a fixed detector that is always located in the OMEGA target chamber. Its low-energy threshold for gamma detection is ~0.5 MeV. This detector was originally built by Atomic Weapons Establishment (AWE) for the purpose of measuring unablated mass in cryogenic implosions via measurement of the 4.44-MeV carbon gamma, which is produced when 14- MeV DT neutrons interact with the carbon shell in a cryogenic DT target. More recently, the DAD has been used in S-factor experiments involving measurement of the 5.5-MeV HD gamma [from the reaction H(D,3He)γ] and the 19.8-MeV HT gamma [from the reaction H(T,4He)γ].
MCP: microchannel plate
Diagram highlighting the physics principles of the gas Cherenkov detector (GCD2) and DAD measurement. Lower energy x rays are mitigated by the shield and housing. Higher-energy gammas, such as the DT γ and carbon 4.4 MeV γ, are able to produce Cherenkov light in the glass (pressure window in the GCD2 and UV-grade SiO2 optical in DAD) and cause ionization near to the photomultipllier tube (PMT) photocathode and microchannel plate.
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2 O. M. Mannion et al., Rev. Sci. Instrum. 92, 033529 (2021).
3 C. Stoeckl et al., Phys. Plasmas 24, 056304 (2017); C. Stoeckl et al. Rev. Sci. Instrum. 85, 11E501 (2014).
4 F. J. Marshall et al., Rev. Sci. Instrum. 92, 033701 (2020).
5 D. Cao et al., Phys. Plasmas 26, 082709 (2019).
6 W. Theobald et al., Rev. Sci. Instrum. 89, 10G117 (2018).
7 M. S. Rubery et al., Rev. Sci. Instrum. 89, 083510 (2018).